Decay of Tails at Equilibrium for Fifo Join the Shortest Queue Networks by Maury Bramson1,

In join the shortest queue networks, incoming jobs are assigned to the shortest queue from among a randomly chosen subset of D queues, in a system of N queues; after completion of service at its queue, a job leaves the network. We also assume that jobs arrive into the system according to a rateαN Poisson process, α < 1, with rate-1 service at each queue. When the service at queues is exponentially distributed, it was shown in Vvedenskaya et al. [Probl. Inf. Transm. 32 (1996) 15–29] that the tail of the equilibrium queue size decays doubly exponentially in the limit as N → ∞. This is a substantial improvement over the case D = 1, where the queue size decays exponentially. The reasoning in [Probl. Inf. Transm. 32 (1996) 15–29] does not easily generalize to jobs with nonexponential service time distributions. A modularized program for treating general service time distributions was introduced in Bramson et al. [In Proc. ACM SIGMETRICS (2010) 275–286]. The program relies on an ansatz that asserts, in equilibrium, any fixed number of queues become independent of one another as N →∞. This ansatz was demonstrated in several settings in Bramson et al. [Queueing Syst. 71 (2012) 247–292], including for networks where the service discipline is FIFO and the service time distribution has a decreasing hazard rate. In this article, we investigate the limiting behavior, as N → ∞, of the equilibrium at a queue when the service discipline is FIFO and the service time distribution has a power law with a given exponent −β, for β > 1. We show under the above ansatz that, as N → ∞, the tail of the equilibrium queue size exhibits a wide range of behavior depending on the relationship between β and D. In particular, if β >D/(D−1), the tail is doubly exponential and, if β <D/(D − 1), the tail has a power law. When β =D/(D − 1), the tail is exponentially distributed.